SoundCore Boost (model A3145) suddenly started discharging itself while turned off

I’ve been using my SoundCore Boost since almost 2 years without problems. Now suddendly, after using the power bank function once again, the charge level display of the device does not turn off any more as it should and as it always did before when the device is shut off and not in use.

The problem: The SoundCore Boost is discharging itself in this state and will be completely empty after a few days – without using it either as BT speaker or as power bank.

This is really awkward and now I can not fully enjoy using it any more: If I don’t use the SoundCore Boost for a week or so, it will be discharged and I would have to wait at least half a day to recharge when I decide to use it. (This is the old version with slow Micro-USB charging port.) Also, Li-Ion batteries usually die quickly when stored fully discharged for some time and that is a really sad prospect!

I only have a guess as to the cause: Of course, the device has a voltage converter to provide 5V at the charging port from 3.7V of the Li-Ion battery. This consumes a bit of electricity and, for some inexplicable reason, doesn’t seem to switch off anymore.